Have you ever been blackmailed, backstabbed, betrayed, bothered, or turned away by someone close to you? I bet you had an urge to try to do the same to them... it's almost an instinct to some to return the favor back at them.
My personal suggestion would be, confront them about it but don't make a big fuss! Talk to them very serious even if you aren't a serious person, look them straight in the eye and tell them straight how you feel. But don't bring it on violence, or curse at them or do anything back at them. If it's someone you don't know too well just ignore them completely, if it's someone you consistently meet up with, talk to them! They are as socialable (and no that's not a word, haha) as anyone else is. Think about the broad perspective, in the long run it will hurt you even more than the person already did. Besides, if you talk thoroughly enough they will feel just as bad as when they 'blackmailed' you!
"An eye for an eye makes the whole world blind."
Enjoy your lives people and relax, remember life is faaaar too beautiful then to waste it on such people or trivial matters. Look at yourself, your hands, your feet, your legs, and at your face. THIS IS WHO YOU ARE!!! This being in front of the mirror, this is where all the glory is... Only you can make your own self to such a state of happiness. Take walks in the sunset, they help too. :)